A lot of suppliers have guidance on selection in their catalogues. For example

https://www.cablecraft.co.uk/cablecraft-catalogue

As Andy says BW and CW are the go-to for SWA in the UK but being aware of other options is advisable particularly if you work in a specialist field. Wiring styles in other countries vary.

In terms of sizing again there are fairly standard tables in catalogues which suggest that there is a fixed relationship between gland size and cable spec, and for smaller commonly used cables experience suggests that is not unreasonable for day-to-day application.

BUT beware that there is no fixed link and for larger or non-standard cables you need to consider the cable dimensions in detail in relation to the glands.

Do not forget to check the manufacturing tolerances over the nominal size: It is not uncommon with larger cables for the tolerance to span two gland sizes, with the upshot that you need to measure the acutal cable on receipt and then order glands, particularly if you buy from a distributor who will themselves be buying from more than one factory. Yes, this can be a pain, which needs to be manged somehow, the solution depending on your project. But I have seen glands which have been tightened all the way up without sealing on the cable, and glands which won't go on securely, despite "correct" on-paper selection based on nominal size.

The same applies to cleats, as an aside, some styles of which are even less tolerant.

A lot of suppliers have guidance on selection in their catalogues. For example

https://www.cablecraft.co.uk/cablecraft-catalogue

As Andy says BW and CW are the go-to for SWA in the UK but being aware of other options is advisable particularly if you work in a specialist field. Wiring styles in other countries vary.

In terms of sizing again there are fairly standard tables in catalogues which suggest that there is a fixed relationship between gland size and cable spec, and for smaller commonly used cables experience suggests that is not unreasonable for day-to-day application.

BUT beware that there is no fixed link and for larger or non-standard cables you need to consider the cable dimensions in detail in relation to the glands.

Do not forget to check the manufacturing tolerances over the nominal size: It is not uncommon with larger cables for the tolerance to span two gland sizes, with the upshot that you need to measure the acutal cable on receipt and then order glands, particularly if you buy from a distributor who will themselves be buying from more than one factory. Yes, this can be a pain, which needs to be manged somehow, the solution depending on your project. But I have seen glands which have been tightened all the way up without sealing on the cable, and glands which won't go on securely, despite "correct" on-paper selection based on nominal size.

The same applies to cleats, as an aside, some styles of which are even less tolerant.